In electric circuit, precise current, especially precise average current, is commonly required to be provided to load. The term “load” hereby includes but not limited to electrical load, battery, and light-emitting diode (LED), etc. Among these loads, the precision control of LED current, especially LED average current, is particularly important. LED current control circuit is applied as an example to introduce the related technology in the following text.
It is well-known that the luminance of LED depends on the average current flowing through the LED. The average current of LED may be set by a current control circuit. FIG. 1 illustrates a LED current control circuit 100, comprising: an input voltage terminal VIN, a switch terminal SW, and a reference ground terminal GND; a fly-wheel circuit 101 comprising an inductor L, a load of LED string and a rectifier coupled in series, wherein the LED string comprises a plurality of LEDs D1-DN, the LED string is further coupled to the input voltage terminal VIN through a sampling resistor RSENSE, and rectifier Rectifier is coupled between the input voltage terminal VIN and the switch terminal SW; a current sense circuit 102 comprising a first resistor R1, a first error amplifier EA1 and a first transistor T1, wherein current sense circuit 102 senses the current flowing through the LED string, and generates a first current signal IFIRST accordingly; a first reference signal REF; a control circuit 103 comprising a second resistor R2, a third resistor R3, a second switch M2, a first comparator C1 and a driver D1, responsive to the first current signal IFIRST and the first reference signal REF, operable to generate a control signal VCON; and a switch M1, coupled between the switch terminal SW and the reference ground terminal GND, turned ON and OFF according to the control signal.
As shown in FIG. 2, when the first switch M1 is turned on (the voltage VSW on the SW terminal is at low level), a loop comprising the input voltage terminal VIN, the inductor L, the load, the switch M1 and the reference ground terminal GND is conducted. The current flowing through the LED string (inductor current IL) gradually increases. By the effect of the current sense circuit 102, ISENSE also gradually increases, and the voltage level on a node B rises up. At this moment, the second switch M2 is turned ON, and the voltage drop on the third resistor R3 and the second switch M2 may be ignored. When the voltage level on node B reaches VREF (the voltage level of REF signal), that is, IFIRST reaches VREF/R2, the first comparator C1 is reversed and generates a low level output signal. The first switch M1 and the second switch M2 are turned OFF (the voltage VSW on the SW terminal is at high level), and the LED string, inductor L, rectifier Rectifier comprises a current loop. The current on the LED load decreases gradually. By the effect of current sense circuit 102, IFIRST also gradually decreases, and the voltage level on node B falls down. When the voltage level on node B falls to VREF, that is, the current IFIRST decreases to VREF/(R2+R3), the comparator P2 is reversed to generate a high level output signal. IFIRST oscillates between a hysteresis voltage [VREF/(R2+R3), VREF/R2], and correspondingly the inductor current IL oscillates between the hysteresis voltage [VREF/(R2+R3), VREF/R2].
Supposing the duty cycle D of the first switch M1 is 50%, when IFIRST reaches the high level peak of the hysteresis voltage VREF/R2, it takes some time for the system to control the switch M1 ON and OFF. Seen in FIG. 2, the TD1 and TD2 are respectively the propagation delay time of the first comparator C1 and the delay time of the driver circuit. TD1 represents the period from the moment when IFIRST reaching VREF/(R2+R3) to the moment when the switch M1 is turned ON, while TD2 represents the period from the moment when IFIRST reaching VREF/R2 to the moment when the switch M1 is turned OFF. ΔITH1 and ΔITH2 are respectively the output current error caused by delays TD1 and TD2. When the duty cycle D is 50%, if TD1=TD2, then ΔITH1=ΔITH2. As a result, the average current flowing through inductor L is still precise.
The variation of input voltage or the number of serially coupled LEDs may lead to the change of duty cycle D. As shown in FIG. 3, supposing the duty cycle D is 10%, it may approximately consider that the up-time of the inductor current is 1/9 of the down time of the inductor current. Thus the current rising rate is 9 times more than the current falling rate. Once TD1=TD2, then ΔITH1=9×ΔITH2. Thus, when the duty cycle D is smaller than 50%, the average inductor current IL may drift to ITH1 and generate an error, so that the average inductor current is higher than the desired value. On the contrary, when the duty cycle D is larger than 50%, the average inductor current IL may drift to ITH2 and also generate an error, so that the average inductor current IL is smaller than the desired value.
Besides the above description, the reasons of generating an error further include but not limited to: the delay, delay difference under different load/inductor current, or non-linearity of current sense circuit 102; the delay, non-linearity or delay difference between rising-edge and falling-edge of control circuit 103; the delay, non-linearity or difference between rising-edge and falling edge of the driver circuit; the varying of the above delays, delay difference or non-linearity caused by the change of duty cycle.